Air conditioning units of this type generally comprise two separate fans, namely a supply fan (blowing fan) and an extract fan (suction fan), and a heat exchanger through which the supply and extract air streams pass, generally in counterflow or cross flow fashion. Conventionally, heat exchangers for use in such air conditioning units comprise a stack of thin plates of metal such as aluminum or copper. Such heat exchangers have minimum resistance to air flow, thus allowing the size of the supply and extract fans, and thus the operating noise level, to be kept to a minimum. However, metal plate heat exchangers are heavy, expensive to manufacture, and the metal (especially aluminiun) can be damaged by corrosive environments such as swimming pools.
One approach to this problem has been to use very thin plastic plates which are light, cheap to manufacture, and still offer very good heat transfer, these plates generally being heavily ribbed to increase the total surface area available for heat exchange. The plastic plates, which may be made from "nylon", PVC or any other suitable plastics material, are also advantageous in that they are resistant to corrosion.
The use of plastic plates has forever created another problem. Because the plates are very thin they tend to flex easily, particularly about axes which are parallel to the ribs. When the plates are stacked together to form a heat exchanger, the plates define a plurality of supply flow paths an extract flow paths which are each made of narrow depth in order to maximise heat recovery i.e. only small spacing apart of the plates in the stack. The narrow flow paths and the flexibility of the plates (together with the differential pressure action of the supply and extraction fans) combine to result in the plate wall faces defining the supply flow path being blown wider apart and the faces defining the extract flow paths being drawn closer together during operation of the unit. This is thought to be due to the pressure differential between the supply and extract fans, and results in reduced extract flow rate and hence lower temperature efficiency. Usually, the supply and extract fans are both located on the outside of the heat exchanger, and one fan operates to blow air into the heat exchanger whilst the other fan operates to draw air from the heat exchanger. An attempt has been made to overcome the problem of the deformation of the flow paths, by placing the supply and extract fans one on either side of the heat exchanger. The supply and extract pressure are then much more balanced, since the two fans are each blowing air into the heat exchanger. However, this arrangement increases the total length of the unit which is unacceptable when the unit is designed to be installed in a wall, and also the location of a fan on the room side of the heat exchanger increases the operating noise level.
An alternative approach is to use a more powerful extract fan to overcome the resistance to flow produced by the narrowing of the extract flow paths. This approach is not acceptable as such an arangement would have a high noise level and higher power consumption, and in addition the stronger extract fan can cause the distortion of the flow paths to increase, thus exacerbating the problem.
It is therefore the aim of the present invention to provide an improved heat exchanger which overcomes the above-mentioned disadvantages.